Process for upgrading an acrylonitrile electrohydrodimerization effluent containing adiponitrile by distillation and alkaline treatment



Feb. 25, 1969 c. R. CAMPBELL ET AL 3,429,783

PROCESS FOR UPGRADING AN ACRYLONITRILE ELECTROHYDRODIMERIZATION EFFLUENT CONTAINING ADIPONITRILE BY DISTILLATION AND ALKALINE TREATMENT Filed Nov. 9. 1967 '0 4 p m V DECANTER WHLSAS 'WAOWHH HBlVM GNV B'HELLIN LHQIHM HV'IHOH'IOW M01 WBLSAS WVAOWBH 3.1.A108l03'13 INVENTORS CHARLES R. CAMPBELL JOHN J. HICKS, JR. MARION J. MATHEWSJZI ATTORNEY United States Patent 3,429,783 PROCESS FOR UPGRADING AN ACRYLONI- TRILE ELECTROHYDRODIMERIZATION EF- FLUENT CONTAINING ADIPONITRILE BY DISTILLATION AND ALKALINE TREATMENT Charles R. Campbell, John J. Hicks, Jr., and Marion J. Mathews III, Pensacola, Fla., assignors to Monsanto Company, St. Louis, Mo., a corporation of Delaware Continuation-impart of application Ser. No. 431,737, Feb. 10, 1965. This application Nov. 9, 1967, Ser. No. 681,834 US. Cl. 203-36 Claims Int. Cl. B01d 3/34, 3/14 ABSTRACT OF THE DISCLOSURE An acrylonitrile electrohydrodimerization effluent containing adiponitrile and 3-hydroxypropionitrile can be substantially upgraded in its content of recoverable constituents by distilling it to remove an overhead fraction containing adiponitrile and at least 25% by weight of 3- hydroxypropionitrile and then contacting the overhead fraction with an aqueous solution of an alkali metal hydroxide, an alkaline earth hydroxide or a quaternary ammonium hydroxide.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our copending application Ser. No. 431,737 which was filed on Feb. 10, 1965, and now abandoned.

BACKGROUND OF THE INVENTION It is known that adiponitrile, a valuable intermediate in the production of polyamides such as polyhexamethylene adipamide, can be prepared by electrohydrodimerizing acrylonitrile in an electrolytic cell having a cathode compartment and anode compartment separated by a cation permeable membrane. Ordinarily, an aqueous solution containing acrylonitrile and an electrolytic salt (e.g. a quaternary ammonium salt) is circulated through the cathode compartment of the cell while an aqueous solution of a strong mineral acid (e.g. sulfuric acid) is circulated through the anode compartment. As electric current is passed through the solutions and the intermediate membrane, acrylonitrile is electrohydrodimerized to adiponitrile at the cathode. The electrolytic reaction also normally produces small amounts of by-products such as propionitrile, 3-hydroxypropionitrile, bis-cyanoethylether, succinonitrile and Z-methylglutaronitrile.

In general, the first step in recovering the desired adiponitrile product is to remove the electrolytic salt from the cathode compartment efiluent. When the salt is a tetraalkylammonium alkylsulfate such as tetramethylammonium methylsulfate, tetraethylammonium ethylsulfate or the like, it can be substantially completely removed by extraction with acrylonitrile and water or by various other methods that are not inconsistent with the overall objectives of the electrohydrodimerization process. Unreacted acrylonitrile and by-product propionitrile are normally thereafter removed by any convenient technique (for example by stripping in conventional fractionation equipment) leaving a tails fraction containing a major proportion and preferably at least about 80% by weight of adiponitrile. In normal operation, the tails fraction also contains a small amount (e.g. 2 to 10% by weight) of 3-hydroxypropionitrile which must be removed in subse quent purification of the adiponitrile. Unfortunately, adiponitrile and 3-hydroxypropionitrile have similar physical properties which prevent their efiicient separation by convenient methods such as distillation.

Before or during the aforementioned removal of acrylonitrile and propionitrile, it is generally desirable to lower the concentration of bis-cyanoethylether in the electrohydrodimerization effluent by reaction with an alkaline catalyst as described in US. 3,280,168. Bis-cyanoethylether is decomposed by the catalyzed reaction to 3-hydroxypropionitrile and acrylonitrile, the latter of which can be conveniently removed with unreacted acrylonitrile and by-product propionitrile from the electrolytic reaction. However, the problem of subsequent 3-hydroxypropionitrile removal is generally further complicated by the additional 3-hydroxypropionitrile that results from bis-cyanoethylether decomposition.

Theoretically, it would be possible to remove 3-hydroxypropionitrile from the electrohydrodimerization effluent by dehydration with the alkaline catalyst by which the bis-cyanoethylether is decomposed. However, it has been found that at the temperatures necessary to dehydrate the relatively small proportion of 3-hydroxypropionitrile in the efiluent at a commercially practical rate, strongly alkaline catalysts such as alkali metal and alkaline earth metal hydroxides may degrade the adiponitrile in the efliuent, and less alkaline catalysts such as quaternary ammonium hydroxides tend to decompose and thereby lose their etfectiveness as dehydrating agents. Accordingly, it is usually preferable to decompose the biscyanoethylether with a quaternary ammonium hydroxide catalyst, strip out the resulting acrylonitrile together with any by-product propionitrile from the electrohydrodimerization reaction, and then distill the remaining etlluent (which typically contains about 2% to 10% by weight of 3-hydroxypropionitrile and at least by weight of adiponitrile) to remove substantially all of the 3-hydroxypropionitrile therefrom. Because of the relative inefificiency inherent in separating adiponitrile and 3-hydroxypropionitrile by distillation, the resulting overhead fraction normally contains a substantial proportion of adiponitrile and, for the same reason, it has appeared that further separation and recovery of constituents of that overhead fraction would be economically impractical. However, in view of the significant value of such constituents, a commercially feasible process to facilitate their recovery is very desirable, and it is an objective of this invention to provide such a process.

SUMMARY OF THE INVENTION It has now been discovered that the aforedescribed ob jective can be achieved by a process which comprises distilling an acrylonitrile electrohydrodimerization eflluent containing adiponitrile and 3-hydroxypropionitrile to re move an overhead fraction containing adiponitrile and at least about 25 by weight of 3-hydroxypropionitrile and then contacting said overhead fraction at a temperature between about and about 225 C. with an aqueous solution containing not less than 0.01%, based on the weight of said fraction, of an alkali metal hydroxide, an alkaline earth metal hydroxide or a quaternary alkyl. ammonium hydroxide in which each alkyl group contains from one to eight carbon atoms. In a specific embodiment of the invention, the overhead fraction is contacted with the aqueous solution for a time suflicient for substantial dehydration of the 3-hydroxypropionitrile to acrylonitrile which is advantageously withdrawn from the mixture as it is produced by the dehydration reaction.

DESCRIPTION OF THE DRAWING AND PRE- FERRED EMBODIMENTS OF THE INVENTION The invention will be more readily understood by reference to the following detailed description taken in conjunction with the accompanying drawing which is a schematic flow diagram representing a specific embodiment of the process of the invention. In the embodiment shown in the drawing, the cathode compartment efiluent from an acrylonitrile electrohydrodimerization cell is fed through line 1 to an electrolyte removal system 2 in which the dimerization electrolyte (usually a quaternary ammonium salt such as tetraalkylarnmonium alkylsulfate in which each alkyl group contains from one to four carbon atoms) is removed by any convenient method such as extraction which, in the case of the aforementioned tetraalkylammonium alkylsulfates, is normally best carried out with water and acrylonitrile. The composition of the cathode compartment eflluent can and normally does vary with changes in the electrolytic process feed composition, electrolysis conditions, etc. However, the following composition may be regarded as typical.

Component: Weight percent Adiponitrile 15 Acrylonitrile 15 Quaternary ammonium sulfate 39 Water 29 Impurities (including reaction lay-products, di-

valent sulfate ions, etc.) 2

The electrolyte removal system 2 can include any suitable apparatus of a batch or continuous type such as one or a series of vessels for mixing and separation of the resulting organic and aqueous phases, e.g. by decantation, one or more towers containing packing or trays suitable for intimately contacting the effiuent and extraction media, or the like. In the embodiment in which the electrolyte is a quaternary ammonium salt and the extraction is carried out with water and acrylonitrile, the lighter desalted efiiuent is withdrawn from system 2 through line 3 and the relatively heavier aqueous phase containing substantially all of the electrolytic salt is withdrawn through line 4.

It is evident that the composition of the desalted effluent in line 3 can vary substantially with the electrolyte removal method employed in system 2 and, when the removal method is extraction, the ratios of extraction media to cathode compartment effluent, etc. However, when the extraction media are water and acrylonitrile, the desalted efiluent is normally saturated with water and ordinarily contains about 65-80% acrylonitrile, 15-25% adiponitrile, 1-10% propionitrile and 1-5% of other electrohydrodimerzation by-products. The following specific composition is typical.

High-boilers (primarily higher molecular weight oligomers of acrylonitrile) 3-hydroxypropionitrile Cyanovaleric acid 0.2 2-methylglutaronitrile 0. 1

The desalted effluent from system 2 is fed through line 3 to a low molecular weight nitrile and water removal system 5 which is preferably a stripping column suitable for distilling off a gaseous overhead mixture containing substantially all of the acrylonitrile, propionitrile and water from the desalted efliuent. If desired, however, the low molecular weight nitriles and water can be alternatively removed by any other convenient method.

As disclosed in US. 3,280,168, it is generally preferable to substantially purify the desalted efliuent of bis-cyanoethylether during the low molecular weight nitrile and water removal, although it can be alternatively carried out at various other points in the adiponitrile recovery process. Utilizing the method of US. 3,280,168 with a stripping column for nitrile and water removal, an alkaline dehydration catalyst (preferably a quaternary ammonium hydroxide) is fed into stripping column 5 through line 6. The overhead stream 8 from stripping column 5 Component: Weight percent Adiponitrile 90.8 High-boilers 3.7 3-hydroxy-propionitrile 3.5 Cyanovaleric acid 0.4 2-cyanocyclopentylideneimine 0.4 2-methylglutaronitrile 0.3 Acrylonitrile 0.3 Bis-cyanoethylether 0.2

In other cases in which a diflferent or no alkaline catalyst is used for bis-cyanoethylether decomposition, the concentrations of adiponitrile and 3-hydroxypropionitrile in the stripper tails fraction can vary substantially, but generally within the ranges of -95% and 2-7%, respectively. Representative compositions can be found in US. 3,280,168.

The stripping column tails are then normally further distilled to remove high-boiling impurities and the aforementioned low-boiling overhead fraction containing 3-hydroxypropionitrile and adiponitrile. Although the order of those two distillation steps can be reversed if desired, they are preferably carried out as shown in the drawing in which the stripper tails from line 7 are fed to a highboiler removal column 9 of any suitable design and therein separated by distillation into a bottoms fraction (withdrawn through line 38) which normally contains 30-90% high-boiling impurities such as oligomers of acrylonitrile, 1060% adiponitrile and 5-15% cyanovaleramide, and an overhead mixture that passes through line 37 to a low boiler removal column 39. That overhead mixture, which is usually -95% by weight of the feed to column 9 and normally contains 88-98% adiponitrile and 2-10% 3- hydroxypropionitrile, is separated by distillation in column 39 into a bottoms fraction (withdrawn through line 40) containing the major portion of the adiponitrile prodnot and an overhead fraction or make which is usually 5-10% by weight of the feed to column 39 but which can vary outside that range depending on the proportion of low-boiling impurities in the column 39 feed, the efiiciency of the separation of adiponitrile and 3-hydroxypropionitrile in column 39, etc. To obtain a column 39 bottoms stream that is essentially pure adiponitrile without driving ofif an unduly large amount of adiponitrile in the overhead fraction, column 39 is desirably operated under a head pressure of 5-50 mm. Hg and preferably 10-20 mm. Hg. The base temperature will be the boiling point of adiponitrile at the base pressure and usually about 200 C. The head temperature is dependent on the degree of separation desired, the amount of low-boiling impurities in the column 39 feed, the overhead fraction take-off rate, etc., and therefore may vary from about to about 180 C. The conditions in column 39 can be adjusted as desired to obtain an overhead fraction containing at least about 25%, preferably at least about 50% and in some cases up to 85% or more by weight of 3-hydroxypropionitrile. The remainder of the overhead fraction is predominantly adiponitrile and preferably includes not more than about 10% of the adiponitrile from the column 39 feed. The following composition represents an exemplary overhead fraction from column 39.

Component: Weight percent 3-hydroxypropionitrile 50.2 Adiponitrile 42.1 Z-methylglutaronitrile 4.4 Succinonitrile 1 .3 Acrylonitrile 0.4 2-cyanocyclopentylideneimine Trace Other miscellaneous low boiling impurities 1.6

Although the column 39 overhead fraction is usually not large in relation to the column 39 bottoms, it is large enough that recovery of valuable constituents therefrom represents a substantial economic advantage, and it is that advantage which is achieved by the present invention. In accordance therewith, and as shown in the drawing, the overhead fraction from column 39 is fed through line to a reactor 12. An aqueous solution of a compound having an alkaline reaction is also fed to reactor 12 in any convenient way such as through line 14 to dehydrate 3- hydroxypropionitrile in the column 39 overhead fraction to lower-boiling acrylonitrile, Alkali metal hydroxides, alkaline earth metal hydroxides and quaternary alkyl ammonium hydroxides (preferably those in which each alkyl group contains from one to eight carbon atoms) can be employed to achieve the desired dehydration reaction. Examples of suitable quaternary alkyl ammonium hydroxides include tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide and the like. However, quaternary ammonium hydroxides tend to decompose at optimum dehydration temperatures in some cases and accordingly, the alkali metal and alkaline earth metal hydroxides are generally prefrered. The most suitable alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Calcium hydroxide and barium hydroxide are examples of suitable alkaline earth metal hydroxides. Especially preferred because of low cost and good resistance to decomposition are sodium hydroxide, potassium hydroxide, calcium hydroxide and barium hydroxide.

The mixture in reactor 12 is heated to between 130 and 225 C. by recirculation through reboiler 17 via lines 15, 18 and 19. For best results, the residence time of the mixture within that temperature range should be at least 0.1 hour. The preferred temperature range is between 180 and 225 C. Reboiler 17 is heated by steam fed through line 22. Condensate is removed via line 24.

Some 3-hydroxypropionitrile dehydration occurs at hydroxide concentrations as low as 0.01 percent by weight of the feed to reactor 12. However, best results are usually obtained with a hydroxide concentration of at least 0.1 percent by weight of reactor feed. The upper concentration limit of a particular hydroxide is dependent on its solubility. When the concentration of a hydroxide becomes greater than its specific solubility in the reaction mixture, undesirable deposition of solids may occur. Therefore, hydroxide concentration in reactor 12 should be held below the level where solidification occurs. In general, it is preferable to feed the alkaline catalyst to reactor 8 in a concentrated form, for example in an aqueous solution containing from 1 up to 45 or more weight percent of the hydroxide.

Acrylonitrile formed by dehydration of the 3-hydroxypropionitrile is vaporized at the reaction temperature and withdrawn as a vapor through line 26, preferably as it is formed by the dehydration reaction. Although not necessary to the process, a sparge of water may be fed to reactor 12 through line 16 to aid in removing acrylonitrile from the liquid reaction mixture in the reactor. The vapor in line 26 is condensed in condenser 27 by cooling with water fed through line 28 and withdrawn through line 30. The resulting condensate flows through line 32 to decanter 34 in which it separates into two layers, a lower aqueous layer withdrawn through line 35 and an upper organic layer withdrawn via line 36, The acrylonitrile product of the dehydration reaction can be efliciently recovered (e.g., by distillation) from both the upper and lower layers and may then be recycled, if desired, for use as electrohydrodimerization feed. Reactor bottoms (withdrawn through line 20) are concentrated in adiponitrile which can also be recovered and refined by conventional methods. When reaction temperatures within the aforedescribed range are employed and the residence time within reactor 12 is regulated so that the mixture is contacted by the alkaline catalyst for up to about 2 hours, preferably for 0.5 to 2 hours, the concentration of adiponitrile in the reactor bottoms in substantially higher than that in the reactor feed (column 39 overhead fraction).

The following specific examples are included to illustrate the process of this invention and should not be regarded as representing any limitation on the manner in which the process can be carried out. Percentages are by weight except where noted otherwise.

Example I An acrylonitrile electrohydrodimerization cathode compartment effluent was treated as described hereinbefore to substantially completely remove the electrolytic (quaternary ammonium) salt, acrylonitrile, propionitrile and water. The remaining electrohydrodimerization effiuent (stripper tails fraction) was distilled in a low boiler removal column to provide an overhead fraction having the following composition.

1120 grams per hour of the overhead fraction having the foregoing composition were fed to a reactor-reboi-ler system together with 270 grams per hour of a 6.6% aqueous solution of sodium hydroxide. The electrically heated reboiler supplied the heat for a one-plate distillation system. It was also equipped with a weir arrangement for continuous withdrawal of reactor bottoms. Overhead vapors from the reactor were condensed in a glass water-cooled condenser. Condensate flowed to a decanter from which the organic layer and the aqueous layer were separately withdrawn. Reboiler temperature was maintained at 200 C. The residence time was 0.5 hour. Head temperature varied between and C. A total of 4400 grams of the low boiler removal column overhead fraction and 970 grams of the sodium hydroxide solution were fed to the system. The reactor overhead vapor produced 1246 grams of organic layer and 1629 grams of aqueous layer. 2497 grams of reactor bottoms were collected. The compositions of the organic layer, aqueous layer and reactor bottoms were as follows.

Reactor Overhead Vapor The procedure of Example I was repeated with the exception that the low boiler removal column overhead fraction was fed at the rate of 2200 grams per hour and a 2% aqueous solution of potassium hydroxide was 7 fed at the rate of 880 grams per hour. The organic layer, aqueous layer and reactor bottoms weighed 599, 773 and 1276 grams, respectively, and had the following compositions.

Reactor Overhead Vapor It can be seen from the foregoing detailed description and examples that by the process of this invention, an acrylonitrile electrohydrodimerization efiiuent comprised predominantly of difiicultly separated components (adiponitrile and 3-hydroxypropionitrile) is conveniently upgraded to provide one stream (reactor 12 bottoms) containing adiponitrile in substantially higher concentration than the reactor feed and a second stream (reactor 12 overhead) containing a high concentration of easily recovered acrylonitrile. Moreover, such upgrading is accomplished by treatment of a distilled overhead fraction containing only a minor proportion of the adiponitrile product of the electrohydrodimerization reaction and, accordingly, the equipment needs and materials handling problems of the present process are significantly less than those which would be presented by carrying out a similar upgrading procedure prior to distillation of the electrohydrodimerization eflluent to remove an overhead frac tion rich in 3-hydroxypropionitrile in accordance with the process of this invention.

Although the process of this invention has been described in specific embodiments, it will be appreciated by those skilled in the art that many modifications and variations thereof may be employed Without departing from the spirit and scope of the invention. Accordingly, it should be understood that the invention is not limited to the embodiments described herein except as is defined in the appended claims.

We claim:

1. A process which comprises contacting an acrylonitrile electrohydrodimerization efiluent containing adiponitrile and bis-cyanoethylether with an alkaline catalyst to decompose said bis-cyanoethylether to acrylonitrile and 3-hydroxypropionitn'le, stripping acrylonitrile from the contacted effluent by distillation, thereafter distilling said efiiuent to remove an overhead fraction containing adiponitrile and 3-hydroxypropionitrile and then contacting said overhead fraction at a temperature between about 130 and about 225 C. with an aqueous solution containing not less than 0.01%, based on the weight of said fraction, of an alkali metal hydroxide, an alkaline earth metal hydroxide or a quaternary alkyl ammonium hydroxide in which each alkyl group contains from one to eight carbon atoms.

2. A process as defined in claim 1, in which the overhead fraction contains at least about by weight of 3-hydroxypropionitrile.

3. A process as defined in claim 1, in which the alkaline catalyst is a quaternary ammonium hydroxide and said overhead fraction is contacted between 180 and 225 C. with an aqueous solution containing not less than 011%, based on the weight of said fraction, of an alkali metal hydroxide or an alkaline earth metal hydroxide.

4. A process as defined in claim 1, in which the overhead fraction contains not more than about 10% of the adiponitrile from said efiluent.

5. A process as defined in claim 1, in which the overhead fraction is contacted with said solution at said temperature for a time sufiicient for substantial dehydration of the 3-hydroxypropionitrile in said fraction to acrylonitrile.

6. A process as defined in claim 5, which further comprises withdrawing acrylonitrvile from said fraction as it is produced by said dehydration.

7. A process as defined in claim 1, in which the aqueous solution contains sodium hydroxide, potassium hydroxide or calcium hydroxide.

8. A process as defined in claim 1, in which the overhead fraction is contacted for 0.5 to 2 hours between 180 and 225 C. with an aqueous solution containing not less than 0.1%, based on the weight of said fraction, of sodium hydroxide, potassium hydroxide or calcium hydroxide.

9. A process which comprises separating an electrolytic salt from an acrylonitrile electrohydrodimerization efiiuent containing adiponitrile, acrylonitrile, 3-hydroxypropionitrile, bis-cyanoethylether and said electrolytic salt, contacting the desalted efiluent with an alkaline catalyst to decompose said bis-cyanoethylether to acrylonitrile and 3-hydroxypropionitrile, stripping acrylonitrile from the contacted efliuent by distillation, thereafter distilling said efliuent to remove an overhead fraction containing adiponitrile and 3-hydroxypropionitrile and then contacting said overhead fraction at a temperature of about to 225 C. with an aqueous solution containing not less than 0.01%, based on the weight of said fraction, of an alkali metal hydroxide, an alkaline earth metal hydroxide or a quaternary alkyl ammonium hydroxide in which each alkyl group contains from one to eight carbon atoms.

10. A process as defined in claim 9, in which the alkaline catalyst is a quaternary ammonium hydroxide and said overhead fraction is contacted between 180 and 225 C. with an aqueous solution containing not less than 0.1%, based on the weight of said fraction, of an alkali metal hydroxide or an alkaline earth metal hydroxide.

References Cited UNITED STATES PATENTS 3,280,168 10/1966' Campbell et a1. 260465.8

NORMAN YUDKOFF, Primary Examiner.

WIL'BUR L. BA'SCO'MB, JR., Assistant Examiner.

US. Cl. X.R. 

